Exhaust gases discharged from thermal power plants, etc., using fossil fuel as an energy source contain a large amount of nitrogen oxide (NOx), and such nitrogen oxide is known to cause acid rain and respiratory diseases. Thus, a variety of techniques for removing nitrogen oxide from exhaust gases are being developed.
FIG. 1 illustrates a conventional exhaust gas denitrification system using selective catalytic reduction (SCR) which is the most widely used to remove nitrogen oxide. With reference to FIG. 1, a conventional exhaust gas denitrification system 1 is configured such that aqueous urea is sprayed into a reaction chamber 13 via a spray unit 124 and thus is converted into gaseous ammonia using waste heat of exhaust gas, so that a gas mixture of exhaust gas and ammonia is fed into a reactor 14. The gas mixture fed into the reactor 14 undergoes a denitrification reaction in the presence of a catalyst, whereby nitrogen oxide is removed from the exhaust gas.
However, in the course of continuously spraying aqueous urea into the reaction chamber 13 via the spray unit 124, the aqueous urea fed into the spray unit 124 is not completely discharged into the reaction chamber 13 but may partially remain behind in the spray unit 124. As such, the remaining aqueous urea is problematic because it may solidify and undesirably block the spray unit 124.
Also, in the course of continuously feeding the aqueous urea into the spray unit 124, the aqueous urea has predetermined viscosity, and thus a dead zone where aqueous urea does not flow but remains still is created in an aqueous urea passage 123 between an outlet of a flow control pump 122 and the spray unit 124. The aqueous urea which does not flow but remains still solidifies, and the solidification thereof is further carried out in the course of continuously supplying the aqueous urea to the spray unit, undesirably blocking the aqueous urea passage 123 due to the solidified urea.
Moreover, in the case where the exhaust gas denitrification system 1 ceases to operate due to a temporary malfunction, the aqueous urea remains behind in the aqueous urea passage 123 or the spray unit 124 and thus solidifies, undesirably blocking the aqueous urea passage 123 or the spray unit 124.